Combustion turbines generally comprise a casing for housing a compressor section, a combustor section and a turbine section. Each one of these sections comprise an inlet end and an outlet end. A combustor transition duct is mechanically coupled between the combustor section outlet end and the turbine section inlet end to direct a working gas from the combustor section into the turbine section.
The working gas is produced by combusting an air/fuel mixture. A supply of compressed air, originating from the compressor section, is mixed with a fuel supply to create a combustible air/fuel mixture. The air/fuel mixture is combusted in the combustor to produce a high temperature and high pressure working gas. The working gas is ejected into the combustor transition duct to direct the working gas flow exiting the combustor into the first stage of the turbine section.
As those skilled in the art are aware, the maximum power output of a gas turbine is achieved by heating the gas flowing through the combustion section to as high a temperature as is feasible. The hot working gas, however, may produce combustor section and turbine section component metal temperatures that exceed the maximum operating rating of the alloys from which the combustor section and turbine section are made and, in turn, may induce premature stress and cracking along various turbomachinary components. In particular, the high firing temperatures generated in the combustion section, combined with the complex geometry of the transition duct, can lead to temperature-limiting levels of stress within the transition duct. Materials capable of withstanding extended high temperature operation are used to manufacture transition ducts, and ceramic thermal barrier coatings may be applied to the base material to provide additional protection. Air cooling may also be provided, such as by utilizing shell air provided from the compressor section to the casing of the combustor section surrounding the transition ducts. For example, cooling air may be routed through cooling passages formed in the transition duct, or it may be impinged onto the outside (cooled) surface of the transition duct, or it may be allowed to pass through holes from the outside of the transition duct to the inside of the duct to provide a barrier layer of cooler air between the combustion air and the duct wall (effusion cooling).
There continues to be a need to improve the cooling of transition ducts to permit operation at higher working gas temperatures, while also reducing or minimizing the cooling air requirement associated with the increased working gas temperatures in order provide improved efficiencies in the output of gas turbine engines.